False affidavit - family law

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Was wondering if it's legal to read another person's sworn affidavit without their consent?

A long story short - my sister was shown someone's affidavit which contains information about her, that could jepardise her career. The information is false, and she is in the process of getting the writer prosecuted. The writer's lawyer was contacted and she informed the police officer / prosecutor that my sister should never have read the affidavit in the first place as it's illegal to share these kinds of documents (it was for a custody hearing), so she would be unable to press any charges.